MAPPING FAULTS AND FRACTURES IN KARST AND INFERRING HYDROGEOLOGICAL PROCESSES AT OAK RIDGE, TN, USING 2-D REFRACTION TOMOGRAPHY

Arrival times from P-wave refraction surveys along Bear Creek Valley on the Oak Ridge Reservation, Tennessee, were inverted to produce velocity tomographic images (tomograms) that show depth and velocity variations within the upper karstic bedrock, which is covered by 0-10 m of unconsolidated overburden. Inverted velocities are consistent with the two distinct bedrock groups: the Nolichucky shale and Maynardville limestone. Distinct low-velocity zones are visible in the velocity tomograms and these zones have the appearance of near-vertical “fracture-like” structures. Some of these low-velocity zones represent previously mapped bedrock faults and fractures, including major strike-slip faults.

Sediments filling bedrock depressions and/or thick zones of highly weathered bedrock along faults and joints produce the low-velocity zones. These zones may be partially or completely saturated, and they probably play an important role in the hydrogeology of the area. Some of these low-velocity zones also lie directly beneath surface streams, suggesting fault control of these streams, including Bear Creek. In many such locations well logs report solutionally-enlarged fractures, cavities, and anomalous water-level fluctuations that cannot be explained by seasonal variations alone.